I see a lot of people talking about how Voldemort would never win a war because of how many muggles there are compared to wizards compared to death eaters (I saw the number 300 thrown out there), the fact that muggles have guns, which you can't Dodge, etc. But you have to consider Voldemort's character. And war tactics for that matter.
For one, Voldemort is very calculated. Once he returned in Order of the Phoenix, he didn't just immediate declare war on the wizarding world with his thirteen or so death eaters. He rallied and bid his time. He knew he needed weapons and numbers if he was going to get anywhere. Voldemort's primary weapon is fear and influence. So the first thing he did was get people afraid, and then he infiltrated the Ministry of Magic, eventually overthrowing it completely, and he did that with only a handful of Death Eaters, because of his tactics which aren't to charge in with guns blazing, but with cunning and deciet, and again, fear. He put the right people on right places.
He'd run his war on the muggles the way he started to in Deathly Hallows - by making it legit. First by eliminating Muggle-borns, many of which wouldn't be afraid to resist Voldemort, but wouldn't think to resist the Ministry of Magic. Over time, he'd use his influence to spread a mindset of hatred towards Muggles and Muggle-borns. In this way, he'd amass numbers. His death eaters would just be his primary strike force, but he'd eventually over many years probably succeed in creating complete hatred for muggles. Then the real war would start.
As for muggles having guns and guns being much more lethal than magic, that's true, but think of the defenses and methods wizards have. Again this wouldn't be a war of weapons, it would be manipulation. His next step would probably be to infiltrate muggle government. Get a wizard into top positions, presidents and prime ministers, or possible get them under the imperius curse. Only then would wizards reveal themselves, at which point the ministry of magic and other wizarding government's would meet with the muggle government's, and suddenly laws would change in favor of wizards. They'd literally trick muggles into making themselves subservient to wizards. And any muggle that didn't like it would have both the wizarding community and now their own muggle military to deal with. Guns would be banned, and anyone that resisted, try fighting a contingency of muggle military flanked by a bunch of wizards under disillusionment charms.
Long story short we'd be screwed, guns or no guns.